The high affinity receptor for IgE ("E-receptor") on mast cells plays a central role in immediate hypersensitivity. Reaction of receptor-bound IgE with polyvalent antigen clusters the receptors and this stimulates a variety of cellular responses characteristic of allergic and inflammatory events. We study the molecular mechanisms by which E-receptors generate such responses. During the past year, we have principally explored the "kinetic proofreading" mechanism which we had previously shown to exist in the mast cell line we use to study the E-receptor. When receptor-initiated signaling is subject to kinetic proofreading, then the degree to which the system proceeds through sequential steps of a signaling cascade is governed by the lifetime of the initial ligand: receptor interaction. Earlier studies revealed evidence for this mechanism. Moreover, in conjunction with our prior observation on the limiting amount of a critical kinase, it provided a basis for the apparent "antagonist" effect of excess weakly binding ligand on the response of the cells to a strongly binding ligand even when the two ligands were not competing for the same ligand binding sites. We then examined the effect of conditions that tend to prolong and intensify the cellular responses. Several novel results were observed the most striking being that even ligands that interact relatively weakly can stimulate certain late events; in particular, the synthesis of mRNAs coding for various mediators. Much of our effort during the past year has been to explore the basis for this apparent paradox. We have postulated (and explored by mathematical modeling) the possibility that certain late signals are responding to intracellular messengers that are not associated with the receptor:ligand complex thereby forming a branch that is no longer constrained by kinetic proofreading. In particular we have monitored intracellular Ca2+ as a candidate for such a messenger. We observed that its increase is effectively stimulated by the low affinity ligand and that this is sufficient to promote the synthesis of the mRNA we are monitoring. In a second study under this topic, we are following up an observation on a critical kinase (Syk) whose phosphorylation is an early consequence of receptor-initiated signaling. One way that would account for the experimental findings would be if the weakly binding ligand could only stimulate the phosphorylation of a subset of tyrosines on Syk. To explore this possibility has been technically challenging but we are continuing to make steady progress in developing the tools to examine this possibility. We have in addition spent considerable effort in preparing a series of transfectants expressing variable amounts of some of the principal early kinases in order to explore quantitatively certain aspects of our model related to the phosphorylation of Syk. Finally, we are close to completing the studies in which we are trying to understand the basis of the large effects of cytoskeletal perturbing agents on the initial events stimulated by the E-receptor. The similarity of E-receptors to other receptors of the immune system (e.g. the clonotypic receptors on T and B lymphocytes), make it likely that the significance of our studies extends beyond the IgE/mast cell system.